Tunable magnetron tube



y 19, 1953 F. H. CRAWFORD TUNABLE MAGNETRON TUBE Filed Jan. 5, 9 6

' INVENTOR. FRANZO H. CRAWFORD A T TORNE Y Patented May 19, 1953 umreo STATES TENJT CFF 2 Claims.

This invention relates generaliy toultrahigh frequency oscillator tubes. More particularly it relates to animprovement in tunable magne' trons for suppressing spurious resonances and maintaining a relatively more uniform Q of the tube;

In the development of oscillators that generate ultra-highfrequencies, numerous" problems have existed. some of the most important problems have been related to spurious oscillations tun-. able magnetrons'. It ismost desirable to solve these problemssothat advantage can be taken of the well known efiiciency'oi this type of tube.

Spurious resonances are due inpart to the addi ti'on ofelements comprisingtuning means. These means, being metallicelemcnts fitted into the resonantcavites ofthe magnetrons and being relatively complex, offer conductive paths to radio frequency currents which often are long enough with respect to certain wave lengths tobecome resonant at corresponding spurious frequencies. Thereasons for changes in Q is that withdraw-- al of the loading slug usually employed to vary self-inductance of'the cavities and thuseffect the frequency of oscillation lessens losses within the tube and raises the Q while the simultaneous insertion of metallic elements used to increase the capacity of the cavities does not addlosses to a commensurate extent.

It is an object of this invention therefore to provide a magnetron tubehavi'ng the advan tages of eniciency and of usefulness which are generally associated with tunable magnetrons, while at the same time it will be less subject to" undesired resonances and to Wide variations of the Q of the tube.

Generally tliis invention comprises a; magnetroneither of the type known to the art as a pluralcavity magnetron or of the type: known as a vane type magnetron, combined with associated elements which can be controllably in connection with the accompanying drawing,- in which: I

Fig; l is a sectional View of a magnetron bodying the principles of this invention, said sectional view being taken along the diameter indicated by the line I-'--l of Fig. 2;

Fig. 2 is a sectional view of the same en'ib'oiiiinent as shown in Fig. 1-, this section eing in a plane parallel to the ends of the cylindrical body of thema netron and at the location shown by line 2 2 in Fig. 1;

Fig. 3 is a sectional view of a vane type magnetron having conventional tuning means-,- said sectional view corresponding in location to the sectional view of 2 and Fig. i is a" sectional view of an embodiment" of this invention as applied to a vane type mag netr'on' and therefore comprising an improve: ment over the magnetron shown in Fig. 3.

Referring now more particularly to Fig. '1 case It is the main body portion of em bodiment of this invention. Approximately midway between its ends H and i2, is a niet'al lic mass [3' which is formed tocontain multiple resonant cavities i4 and to contain acenter cavity I 5 con'iprising the inter elcctrode space through which electrons travel from cathode it toward cavities I4. The location of cathode it is conventional, as are the design andarrang'e ment of its heater ll. If no other parts were to be added to the structure thus far referred to, this device in the presence of the proper magnetic field and with proper heating and polarizing voltages, would be" a fixed frequency magnetron oscillator. The parts tobe described hereinafter are parts adapted to make it turn able, and include the novel" features of this in vention.

Referring now to- Fig. 2, it will be observed that: cavities M are connected to central cavity I5 by slots |"8.- As is well known inthe art", the gaps or openings comprising slots l8=' have certainvalues of capacitance which are deter-" mining factors in the frequency of oscillation of these resonant cavities. Likewise, it is well known in the" art that the portions of cavities Hi which comprise complete cylinders; except" for theamounts removed to form slots l8, have'relatively lumped values of self-inductance which also have a controlling eifect on the operating. frequency of these resonant cavities. To be sure; there are both self-inductance and capacitance which, in addition to these easily discernible lumped parameters, are distributed'through such resonant cavities and are sometimes present in the least obvious of places. However, relatively lumped values do exist in the areas described.

Dielectric blocks 19 are mechanically supported by a structure, which will be described hereinafter, so that they can be inserted into slots l8 or extracted therefrom. It is obvious that the presence of dielectric blocks 19 between the faces of-slots l8 would markedly affect the value of capacitance existing across these slots especially if blocks l9 have a high value of dielectric constant. Likewise the presence or absence of metallic slugs 20 in the center of the inductive loop formed by cavity M has a marked effect upon the self-inductance of those loops. Accordingly, metallic slugs 2e are likewise supported by a structure, to be described hereinafter, in such a manner that they can be inserted into or withdrawn from cavity l4,

Referring now again to Fig. 1, it will be seen that dielectric blocks I9 are connected at their upper ends to rods 2| and that metallic slugs 20 are connected at their upper ends to rods 22. Rods 2| and rods 22 are part of the means used -to control the positions of these elements. In length rods 2| are shorter than rods 22. This difference in length is such that the ends of dielectric blocks l9'occur where the beginnings of metallic slugs 20 occur. Because of this construction it is obvious that when metallic slugs are fully within cavity I4, dielectric blocks are fully outside of slots IB, and vice versa. The upper ends of all rods 2| and all rods 22 are fastened into structure 23 which has substantially the shape of a large washer whose outer diameter is approximately as long as the outer diameter of the magnetron and whose inner diameter is approximately the same as that of inner cavity H3. The top ends of rods 2! and of rods 22, that is those ends which are not connected to dielectric blocks l9 or to metallic slugs 20 respectively, are fastened into structure 23 by any suitable means such as Welding, brazing, or silver soldering. The four pairs of rods, each of which pair consists of a rod 2i and a rod 22, are free to move into and out of case It through four holes 24. Each of the four holes 24 is positioned approximately above a cavity 14 and is sufiiciently larger than said cavity M so that each pair of rods consisting of a rod 2| and a rod 22 fit through holes 24 without touching case II] at any point. Fastened above holes 24 with airtight junctions are four Sylphons 25. An end of each of these metallic bellows-like structures is fastened to case it in the position described above and the other ends are fastened to structure 23. In this way rods 2! and rods 22 are free to be pushed into or withdrawn from case l while at the same time a high vacuum can be and is preserved within case l0 and, of course, within Sylphons 25. Any suitable means can be used for moving structure 23. Such means could, for example, take the form of lever means or of screw means, and they could be designed in any of a number of conventional ways.

The foregoing portion of this application has disclosed the physical structure of this embodiment of this invention. The principles governing its operation will be set forth below.

Elements similar to metallic slugs 29 have been used in tunable magnetrons in prior art. They can be looked upon as single turn inductive loops which are short circuited and which can be coupled with the inductance of cavities M by inserting them within these cavities. It is 4 well known in the art that the presence of a short circuited single turn slug coupled to an inductance decreases the self-inductance thereof and that said self-inductance can be considered as changing its inductance value in the same way as the primary of a transformer which has its secondary connected to a low impedance load. As was explained abovetheapresence of the dielectric blocks I9 has-a well known effect upon the capacitance of these resonant cavities.

.Their use in producing this effect has advantages over the prior art use of metallic inserts. Metallic inserts, by lessening the air gap of slots l8, also increase the capacity of these slots. However they offer conductive current paths which are long enough with respect to certain wave-lengths to give rise to spurious resonances. The use of dielectric material, while it retains the ability to vary capacitance, avoids these long conductive current paths.

Since metallic slugs 20 and dielectric blocks 19 are arranged so that slugs 20 are withdrawn at the same time that dielectric blocks 19 are inserted, and vice versa, it follows that when the structure is moved so as to compress or extend Sylphons 25, the inductance and capacitance with either increase simultaneously or willdecrease simultaneously, and accordingly the L-C product can increase rapidly or decrease rapidly.

Moreover the Q of the magnetron tends to be relatively constant over the tunable range of frequency when this arrangement is used, since, when losses due to the insertion of blocks 19 progressively increase and lower the Q of cavities M,losses due to thewithdrawal of metallic slugs 20 progressively decrease and raise the Q. This same compensating action applies when the direction of adjustment is reversed. However this is not true to any comparable extent where the capacity-varying means are metallic inserts such as the means known to the prior art. Metallic inserts when placed in an electric field do not cause large losses and do not compensate for the withdrawal of inductance loading slugs.

Referring to Fig. 4, it will be seen that this improvement is readily adaptable to a vane type of magnetron. A conventional vane type magnetron is shownin Fig. 3 and the only difference between that magnetron and the improved one of Fig. 4 is that elements 21, in the improved type, consist of dielectric material whereas in Fig. 3 the corresponding elements 28 are metallic. In the vane type of magnetron the portions of the resonant cavities which open into the central cavity containing the cathode and which have the form of openings between adjacent vanes are capacitive areas corresponding to slots l8 of Fig. 2. The use of dielectric blocks in these spaces has the same effect of increasing the lumped capacity in this area as that effect already described above. Accordingly dielectric elements 2! in Fig. 4 are indicated with positions generally corresponding to the positions of dielectric blocks IQ of Figs. 1 and 2. Metallic slugs 20 of Figs.3 and 4 situated in the inductive area of the cavities of this vane type magnetron are in all respects the same as metallic slugs 20 in Figs. 1 and 2 and perform the same function.

It will be seen-that the magnetrons herein set forth are capable of being tuned over a wide range of frequency at the same time that they are less prone to spurious resonances and to wide variationsin Q than are conventional tuned magnetrons.

It will be obvious to those skilled in the art that no particular precise form of inserts or slugs is necessary and that no particular form of control mechanism, such as that shown in Fig. 1, need be employed for varying their positions within the cavities. Dielectric blocks having different values of dielectric constant might readily be employed for specific applications, and these blocks might be tapered or deformed in shape so that the rate of change of frequency for a given movement of the control elements would follow some desired pattern.

While there has been described what is at present considered to be the preferred embodiment of this invention it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention, and accordingly I claim all such modifications that fall within the spirit and scope of the hereinafter appended claims.

The invention claimed is:

1. A magnetron comprising a cylindrical cathode, a plurality of cavity resonators concentrically disposed around said cathode and each having an open end facing said cathode, a case enclosing said cavity resonators, and an axially expandable Sylphon means connected to said case, said Sylphon means being disposed perpendicular to the axis of said cathode, frequency adjusting means including a plurality of inductance loading conductive elements, each centrally disposed in one of said cavity resonators, and a plurality of dielectric insert capacity-varying elements, each extending substantially across the open end of one of said cavity resonators, all of said inductance loading elements and all of said dielectric inserted elements being adjustably supported within said resonators of said magnetron by said Sylphon means, whereby the amounts that the inductance loading elements 6 and said dielectric insert elements are inserted into said resonators are simultaneously adjustable and whereby the frequency of oscillation cally, disposed around said cathode and each having an open end facing said cathode, a case enclosing said cavity resonators, frequency adjusting means including an inductance loading conductive element and a dielectric capacityvarying element, a flexible Sylphon means connected to said case and disposed perpendicular to the axis of said cathode for adjustably supporting both said loading element and said dielectric insert element within one of said cavity resonators, said conductive element being centrally disposed in said one of said cavity resonators and said dielectric element extending substantially across the open end of said one cavity resonator, whereby the amount that said inductance loading element and said dielectric element are inserted or withdrawn from said resonator is adjustable and whereby the frequency of oscillation of said cavity resonator may be adjusted over a range of frequencies and spurious resonances minimized.

FRANZO H. CRAWFORD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,408,234 Spencer Sept. 24, 1946 2,422,465 Bondley June 17, 1947 2,449,794 Steele Sept. 21, 1948 2,482,541 Hall et a1 Sept. 20, 1949 2,512,901 Litton June 2'7, 1950 

